Hydrocarbons, such as petroleum products, have been recovered from underground media, such as oil shale, tar sand, and ground water contamination, using a variety of techniques, including heat and chemical treatment. As is well known in the art, heating the petroleum products reduces their viscosity, facilitating extraction from the medium. Various techniques have been proposed for heating the petroleum products. For instance, a geologic formation can be heated via electrodes deployed in the ground using resistance heating. As an alternative, the geologic formation can be heated by steam that is either delivered to the geologic formation or formed within the geologic formation.
Microwave energy can also be used to generate heat for extracting petroleum products from an underground medium. Generally, microwave techniques use an elongate antenna that is located below ground level, typically within a borehole, at the site where heating is desired. A radio frequency (RF) generator, such as a magnetron or a klystron generator, generates an RF signal, which typically contains microwave energy. A coaxial transmission line or other waveguide transmits the RF signal from the RF generator to the antenna, which radiates the RF signal to the surrounding environment, i.e., the underground medium.
The antenna delivers microwave energy to the underground medium, heating the petroleum products. As a result, the viscosity of the hydrocarbons is lowered, which in turn reduces the pumping power involved in extracting the petroleum products. Further, the mobility of the petroleum products in the medium is increased relative to the mobility of water in the medium, reducing the amount of water that is extracted by pumping.
The use of microwave energy to heat and remove hydrocarbons from other environments, such as oil-based emulsions, has also been proposed. For example, in some conventional techniques, a hydrocarbon and water emulsion flows through a microwave cavity having emulsion flow chambers. These chambers, in combination with a microwave waveguide, form a resonant chamber within which microwave energy reflects to treat the flowing emulsion.
While microwave treatment of emulsions can separate the emulsions to some degree, certain drawbacks of some conventional approaches limit the effectiveness of those approaches. For example, the heat generated in the process may be sufficient to ignite nearby materials, such as explosive gas byproducts or the hydrocarbons themselves. Cooling systems or explosion suppression systems are often required to reduce the likelihood of explosion. One technique to suppress ignition and reduce the likelihood of explosion is disclosed in U.S. Pat. No. 5,829,528, issued Nov. 3, 1998 to Uthe, entitled IGNITION SUPPRESSION SYSTEM FOR DOWN HOLE ANTENNAS, the disclosure of which is hereby incorporated herein by reference in its entirety.
Further, in approaches in which microwave energy is emitted from a source above the emulsion, the microwave energy often does not penetrate adequately deeply to treat the entire emulsion. Only a portion of the emulsion, such as a surface layer, is heated effectively, potentially resulting in inefficient demulsification. Even in approaches in which microwave energy is applied via an antenna inserted in a borehole, many antennas do not radiate the microwave energy effectively. As a result, such antennas typically consume relatively large amounts of power, leading to high costs. In addition, a substantial portion of the microwave energy not radiated is internally reflected within the antenna, potentially resulting in heating of the antenna itself and an elevated risk of combustion as described above.